Apparatus and method for wound volume measurement

ABSTRACT

A method and apparatus for measuring a volume of a wound are described, the method comprising the steps of: applying a dressing over a wound, the volume of which is to be measured, the dressing including at least a sealing drape over the wound so as to create a sealed wound cavity; creating a vacuum in said wound cavity by vacuum pump means so as to produce a predetermined vacuum in the wound cavity; measuring a volume of air extracted from said wound cavity in producing said predetermined vacuum; and, calculating a volume of said wound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional of U.S. application Ser. No. 12/746,492,filed Jun. 4, 2010, and issued as U.S. Pat. No. 8,814,841, which is aU.S. National Phase of the PCT International Application No.PCT/GB2008/050917, filed Oct. 7, 2008, designating the U.S. andpublished on Jun. 11, 2009 as WO 2009/071924, which claims priority toGreat Britain Patent Application No. 0723855.3 filed Dec. 6, 2007. Thedisclosures of all these prior applications are incorporated byreference in their entirety and should be considered a part of thisspecification.

BACKGROUND

1. Field

The present invention relates to apparatus and a method for themeasurement of wound volume to assess progress of wound healingparticularly, though not exclusively, during topical negative pressure(TNP) therapy.

2. Background

There is much prior art available relating to the provision of apparatusand methods of use thereof for the application of TNP therapy to woundstogether with other therapeutic processes intended to enhance theeffects of the TNP therapy. Examples of such prior art include thoselisted and briefly described below.

TNP therapy assists in the closure and healing of wounds by reducingtissue oedema; encouraging blood flow and granulation of tissue;removing excess exudates and may reduce bacterial load and thus,infection to the wound. Furthermore, TNP therapy permits less outsidedisturbance of the wound and promotes more rapid healing.

In our co-pending International patent application, WO 2004/037334,apparatus, a wound dressing and a method for aspirating, irrigating andcleansing wounds are described. In very general terms, this inventiondescribes the treatment of a wound by the application of topicalnegative pressure (TNP) therapy for aspirating the wound together withthe further provision of additional fluid for irrigating and/orcleansing the wound, which fluid, comprising both wound exudates andirrigation fluid, is then drawn off by the aspiration means andcirculated through means for separating the beneficial materials thereinfrom deleterious materials. The materials which are beneficial to woundhealing are recirculated through the wound dressing and those materialsdeleterious to wound healing are discarded to a waste collection bag orvessel.

In our co-pending International patent application, WO 2005/04670,apparatus, a wound dressing and a method for cleansing a wound usingaspiration, irrigation and cleansing wounds are described. Again, invery general terms, the invention described in this document utilisessimilar apparatus to that in WO 2004/037334 with regard to theaspiration, irrigation and cleansing of the wound, however, it furtherincludes the important additional step of providing heating means tocontrol the temperature of that beneficial material being returned tothe wound site/dressing so that it is at an optimum temperature, forexample, to have the most efficacious therapeutic effect on the wound.

In our co-pending International patent application, WO 2005/105180,apparatus and a method for the aspiration, irrigation and/or cleansingof wounds are described. Again, in very general terms, this documentdescribes similar apparatus to the two previously mentioned documentshereinabove but with the additional step of providing means for thesupply and application of physiologically active agents to the woundsite/dressing to promote wound healing.

The content of the above references is included herein by reference.

However, the above apparatus and methods are generally only applicableto a patient when hospitalised as the apparatus is complex, needingpeople having specialist knowledge in how to operate and maintain theapparatus, and also relatively heavy and bulky, not being adapted foreasy mobility outside of a hospital environment by a patient, forexample.

Some patients having relatively less severe wounds which do not requirecontinuous hospitalisation, for example, but whom nevertheless wouldbenefit from the prolonged application of TNP therapy, could be treatedat home or at work subject to the availability of an easily portable andmaintainable TNP therapy apparatus.

One particular area of wound therapy which is desirable is to monitorthe volume of a wound during its treatment such as, for example, atdressing change time so as to be able to quantify the healing process.However, there are presently no simple and/or accurate tools with whichto make this assessment of wound volume.

Methods which have been used have include measuring the length, widthand depth of a wound and making some assessment from the dimensions butthis is very inaccurate; Taking a tracing around the wound, calculatingthe area and taking depth measurements; filling the wound with a fillerand measuring the wound volume based on a known density of the filler;and photographic methods. All of these prior methods involve makingcalculated guesses resulting in inaccurate volume figures or are timeconsuming or are rather impractical.

Desirably, wound volume should be measured when a dressing is changedat, for example, every few days so as to keep a continuous record ofprogress.

Whatever method was used it would result in the figures having to bestored in a file, for example, and generally not with the patient beingtreated.

SUMMARY

It is an aim of the present invention to at least partly mitigate theabove-mentioned problems.

According to a first aspect of the present invention there is provided amethod of measuring a volume of a wound, the method comprising the stepsof: applying a dressing over a wound, the volume of which is to bemeasured, the dressing including at least a sealing drape over the woundso as to create a sealed wound cavity; creating a vacuum in said woundcavity by vacuum pump means so as to produce a predetermined vacuum inthe wound cavity; measuring a volume of air extracted from said woundcavity in producing said predetermined vacuum; and, calculating a volumeof said wound.

According to a second aspect of the present invention there is providedapparatus for measuring the volume of a wound, the apparatus comprising:a dressing covering and sealing the wound to form a cavity over thewound; an aspiration conduit leading from the wound cavity to a wastecanister; a vacuum source; and, flow measuring sensor means.

In one embodiment of the present invention, the volume of air may bemeasured by signals received from flow sensor means in an apparatuscontrol system.

Desirably, the wound volume measurement may be made at a time ofdressing change and with an empty waste canister.

Desirably, the wound may be dressed in the same manner as closely aspossible each time so as to minimise variables due to differences in thetype and degree of packing of the wound, for example. For example, thewound may be packed with suitable filler material (if large enough andif appropriate) such as gauze, foam or any other type of fillerappropriate to the wound and the sealing drape may be applied such thatwhen the filler is compressed by the applied vacuum to the predeterminedvacuum pressure then the sealing drape may be flush, for example, withthe patient's sound flesh surrounding the wound. This is explanation ismerely exemplary but is intended to emphasize the desirability of aconsistent manner of wound dressing.

The method of the present may be applied by apparatus for the provisionof TNP therapy to a patient in almost any environment. The apparatus islightweight, may be mains or battery powered by a rechargeable batterypack contained within a device (henceforth, the term “device” is used toconnote a unit which may contain all of the control, power supply, powersupply recharging, electronic indicator means and means for initiatingand sustaining aspiration functions to a wound and any further necessaryfunctions of a similar nature). When outside the home, for example, theapparatus may provide for an extended period of operation on batterypower and in the home, for example, the device may be connected to themains by a charger unit whilst still being used and operated by thepatient.

The overall apparatus of which the present invention is a partcomprises: a dressing covering the wound and sealing at least an openend of an aspiration conduit to a cavity formed over the wound by thedressing; an aspiration tube comprising at least one lumen therethroughleading from the wound dressing to a waste material canister forcollecting and holding wound exudates/waste material prior to disposal;and, a power, control and aspiration initiating and sustaining deviceassociated with the waste canister.

The dressing covering the wound may be any type of dressing normallyemployed with TNP therapy and, in very general terms, may comprise, forexample, a semi-permeable, flexible, self-adhesive drape material, as isknown in the dressings art, to cover the wound and seal with surroundingsound tissue to create a sealed cavity or void over the wound. There mayaptly be a porous barrier and support member in the cavity between thewound bed and the covering material to enable an even vacuumdistribution to be achieved over the area of the wound. The porousbarrier and support member being, for example, a gauze, a foam, aninflatable bag or known wound contact type material resistant tocrushing under the levels of vacuum created and which permits transferof wound exudates across the wound area to the aspiration conduit sealedto the flexible cover drape over the wound.

The aspiration conduit may be a plain flexible tube, for example, havinga single lumen therethrough and made from a plastics material compatiblewith raw tissue, for example. However, the aspiration conduit may have aplurality of lumens therethrough to achieve specific objectives relatingto the invention. A portion of the tube sited within the sealed cavityover the wound may have a structure to enable continued aspiration andevacuation of wound exudates without becoming constricted or blockedeven at the higher levels of the negative pressure range envisaged.

It is envisaged that the negative pressure range for the apparatusembodying the present invention may be between about −50 mmHg and −200mmHg (note that these pressures are relative to normal ambientatmospheric pressure thus, −200 mmHg would be about 560 mmHg inpractical terms). Aptly, the pressure range may be between about −75mmHg and −150 mmHg. Alternatively a pressure range of up to −75 mmHg, upto −80 mmHg or over −80 mmHg can be used. Also aptly a pressure range ofbelow −75 mmHg could be used. Alternatively a pressure range of over−100 mmHg could be used or over −150 mmHg.

The aspiration conduit at its distal end remote from the dressing may beattached to the waste canister at an inlet port or connector. The devicecontaining the means for initiating and sustaining aspiration of thewound/dressing may be situated between the dressing and waste canister,however, in a preferred embodiment of the apparatus embodying thepresent invention, the device may aspirate the wound/dressing via thecanister thus, the waste canister may preferably be sited between thewound/dressing and device.

The aspiration conduit at the waste material canister end may preferablybe bonded to the waste canister to prevent inadvertent detachment whenbeing caught on an obstruction, for example.

The canister may be a plastics material moulding or a composite unitcomprising a plurality of separate mouldings. The canister may aptly betranslucent or transparent in order to visually determine the extent offilling with exudates. However, the canister and device may in someembodiments provide automatic warning of imminent canister fullcondition and may also provide means for cessation of aspiration whenthe canister reaches the full condition.

The canister may be provided with filters to prevent the exhaust ofliquids and odours therefrom and also to prevent the expulsion ofbacteria into the atmosphere. Such filters may comprise a plurality offilters in series. Examples of suitable filters may comprise hydrophobicfilters of 0.2 μm pore size, for example, in respect of sealing thecanister against bacteria expulsion and 1 μm against liquid expulsion.

Aptly, the filters may be sited at an upper portion of the wastecanister in normal use, that is when the apparatus is being used orcarried by a patient the filters are in an upper position and separatedfrom the exudate liquid in the waste canister by gravity. Furthermore,such an orientation keeps the waste canister outlet or exhaust exit portremote from the exudate surface.

Aptly the waste canister may be filled with an absorbent gel such asISOLYSEL (trade mark), for example, as an added safeguard againstleakage of the canister when full and being changed and disposed of.Added advantages of a gel matrix within the exudate storing volume ofthe waste canister are that it prevents excessive movement, such asslopping, of the liquid, minimises bacterial growth and minimisesodours.

The waste canister may also be provided with suitable means to preventleakage thereof both when detached from the device unit and also whenthe aspiration conduit is detached from the wound site/dressing.

The canister may have suitable means to prevent emptying by a user(without tools or damage to the canister) such that a full or otherwiseend-of-life canister may only be disposed of with waste fluid stillcontained.

The device and waste canister may have mutually complementary means forconnecting a device unit to a waste canister whereby the aspirationmeans in the device unit automatically connects to an evacuation port onthe waste canister such that there is a continuous aspiration path fromthe wound site/dressing to an exhaust port on the device.

Aptly, the exhaust port from the fluid path through the apparatus isprovided with filter means to prevent offensive odours from beingejected into the atmosphere.

In general terms the device unit comprises an aspirant pump; means formonitoring pressure applied by the aspirant pump; a flowmeter to monitorfluid flow through the aspirant pump; a control system which controlsthe aspirant pump in response to signals from sensors such as thepressure monitoring means and the flowmeter, for example, and whichcontrol system also controls a power management system with regard to anon-board battery pack and the charging thereof and lastly a userinterface system whereby various functions of the device such aspressure level set point, for example, may be adjusted (includingstopping and starting of the apparatus) by a user. The device unit maycontain all of the above features within a single unified casing.

When the dressing is applied to the wound a volume of air is trappedunder the sealing drape in the wound cavity. When the vacuum source suchas a vacuum pump, for example, is started the wound cavity is evacuatedto a predetermined vacuum such as by 0.1 of an atmosphere (i.e. about 76mmHg below atmospheric pressure). In this case 0.1 of the volume of airin the wound cavity is removed and which is measured by the flow sensormeans such as a flowmeter, for example, in the control and monitoringsystem. The volume of air removed will be proportional to the actualvolume of the wound. The volumes of other parts of the apparatus such asthe aspiration conduit, the waste canister, the vacuum pump and the flowconduits in the apparatus leading to the flow sensor may be measuredand/or are known and constant factors for which a correction factor maybe incorporated in memory means in the control system software.

Once the desired vacuum level is achieved and is steady at that level,the various factors may be computed by the software in the controlsystem to calculate the wound volume. It is important that the desiredvacuum is at a steady state and the vacuum source such as a vacuum pumpmay be running slowly or intermittently to achieve such a steady statedue, for example, to a leak into the wound cavity through or around thesealing drape. The software may contain appropriate data relating to thevacuum pump operating regime to calculate the steady state leak rate andto enable the control and monitoring system to apply a suitablecorrection factor to allow for a steady state leak rate. Thus, when thewound cavity has achieved a steady state when the vacuum level pressureis at the desired value, what the flow sensor is reading is the actualleak rate which may be used by the software to compute the correctionfactor applicable.

Different wound filling materials may have different compressibilityfrom each other. Gauze, for example, is made from material fibres whichthemselves are virtually incompressible at the levels of vacuum underconsideration in the present invention; foam materials, however, aremuch more compressible and thus, correction factors need to be made toallow for the type of wound filling material in use. This may be easilyachieved by test cavities of known volumes used to calibrate theapparatus for different filling materials and calculate appropriatecorrection factors.

As noted above it is desirable that at a time of dressing change thewaste canister is empty since its empty volume forms part of thecorrection factor incorporated into the software. However, it is notnecessary that the waste canister be empty so long as its free space isknown so that a suitable factor may be entered into the control andmonitoring system such as by a key pad associated with the apparatusdevice, for example.

In the present invention the device may have means such as LED displaymeans, for example, of displaying and storing the volume of the wound asmeasured at each stage such as at each dressing change time, forexample, so that a record exists of the progress of wound healing.Alternatively, the device may have an output to a separate displayand/or recording device where data may be held and/or displayed.

In view of the fact that the device unit contains the majority of theintrinsic equipment cost therein ideally it will also be able to surviveimpact, tolerate cleaning in order to be reusable by other patients.

In terms of pressure capability the aspiration means may be able toapply a maximum pressure drop of at least −200 mmHg to a woundsite/dressing. The apparatus is capable of maintaining a predeterminednegative pressure even under conditions where there is a small leak ofair into the system and a high exudate flow.

The pressure control system may prevent the minimum pressure achievedfrom exceeding for example −200 mmHg so as not to cause undue patientdiscomfort. The pressure required may be set by the user at a number ofdiscreet levels such as −50, −75, −100, −125, −150, −175 mmHg, forexample, depending upon the needs of the wound in question and theadvice of a clinician. Thus suitable pressure ranges in use may be from−25 to −80 mmHg, or −50 to −76 mmHg, or −50 to −75 mmHg as examples. Thecontrol system may also advantageously be able to maintain the setpressure within a tolerance band of +/−10 mmHg of the set point for 95%of the time the apparatus is operating given that leakage and exudationrates are within expected or normal levels.

Aptly, the control system may trigger alarm means such as a flashinglight, buzzer or any other suitable alarm means when various abnormalconditions apply such as, for example: pressure outside set value by alarge amount due to a gross leak of air into system; duty on theaspiration pump too high due to a relatively smaller leakage of air intothe system; pressure differential between wound site and pump is toohigh due, for example, to a blockage or waste canister full.

The apparatus of the present invention may be provided with a carry caseand suitable support means such as a shoulder strap or harness, forexample. The carry case may be adapted to conform to the shape of theapparatus comprised in the joined together device and waste canister. Inparticular, the carry case may be provided with a bottom opening flap topermit the waste canister to be changed without complete removal of theapparatus form the carry case.

The carry case may be provided with an aperture covered by adisplaceable flap to enable user access to a keypad for varying thetherapy applied by the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more fully understood,examples will now be described by way of illustration only withreference to the accompanying drawings, of which:

FIG. 1 shows a generalised schematic block diagram showing a generalview of an apparatus and the constituent apparatus features thereof;

FIG. 2 shows a similar generalised schematic block diagram to FIG. 1 andshowing fluid paths therein;

FIG. 3 shows a generalised schematic block diagram similar to FIG. 1 butof a device unit only and showing power paths for the various powerconsuming/producing features of the apparatus;

FIG. 4 shows a similar generalised schematic block diagram to FIG. 3 ofthe device unit and showing control system data paths for controllingthe various functions and components of the apparatus;

FIG. 5 shows a perspective view of an apparatus;

FIG. 6 shows a perspective view of an assembled device unit of theapparatus of FIG. 5;

FIG. 7 shows an exploded view of the device unit of FIG. 6;

FIG. 8 shows a partially sectioned side elevation view through theinterface between a waste canister and device unit of the apparatus;

FIG. 9 shows a cross section through a waste canister of the apparatusof FIGS. 5 to 8;

FIG. 10 shows a schematic cross section of a wound, the volume of whichis to be measured, the wound having apparatus for the application of TNPtherapy thereto;

FIG. 11 shows a graph of flow vs time of an example of wound volumemeasurement; and

FIG. 12 which shows a graph of flow against pressure in the woundcavity.

DETAILED DESCRIPTION

Referring now to FIGS. 1 to 4 of the drawings and where the same orsimilar features are denoted by common reference numerals.

FIG. 1 shows a generalised schematic view of an apparatus 10 of aportable topical negative pressure (TNP) system. It will be understoodthat embodiments of the present invention are generally applicable touse in such a TNP system. Briefly, negative pressure wound therapyassists in the closure and healing of many forms of “hard to heal”wounds by reducing tissue oedema; encouraging blood flow and granulartissue formation; removing excess exudate and may reduce bacterial load(and, therefore, infection). In addition the therapy allows for lessdisturbance of a wound leading to more rapid healing. The TNP system isdetailed further hereinafter but in summary includes a portable bodyincluding a canister and a device with the device capable of providingan extended period of continuous therapy within at least a one year lifespan. The system is connected to a patient via a length of tubing withan end of the tubing operably secured to a wound dressing on thepatient.

More particularly, as shown in FIG. 1, the apparatus comprises anaspiration conduit 12 operably and an outer surface thereof at one endsealingly attached to a dressing 14. The dressing 14 will not be furtherdescribed here other than to say that it is formed in a known mannerfrom well know materials to those skilled in the dressings art to createa sealed cavity over and around a wound to be treated by TNP therapywith the apparatus of the present invention. The aspiration conduit hasan in-line connector 16 comprising connector portions 18, 20intermediate its length between the dressing 14 and a waste canister 22.The aspiration conduit between the connector portion 20 and the canister22 is denoted by a different reference numeral 24 although the fluidpath through conduit portions 12 and 24 to the waste canister iscontinuous. The connector portions 18, 20 join conduit portions 12, 24in a leak-free but disconnectable manner. The waste canister 22 isprovided with filters 26 which prevent the escape via an exit port 28 ofliquid and bacteria from the waste canister. The filters may comprise a1 μm hydrophobic liquid filter and a 0.2 μm bacteria filter such thatall liquid and bacteria is confined to an interior waste collectingvolume of the waste canister 22. The exit port 28 of the waste canister22 mates with an entry/suction port 30 of a device unit 32 by means ofmutually sealing connector portions 34, 36 which engage and sealtogether automatically when the waste canister 22 is attached to thedevice unit 32, the waste canister 22 and device unit 32 being heldtogether by catch assemblies 38, 40. The device unit 32 comprises anaspirant pump 44, an aspirant pressure monitor 46 and an aspirantflowmeter 48 operably connected together. The aspiration path takes theaspirated fluid which in the case of fluid on the exit side of exit port28 is gaseous through a silencer system 50 and a final filter 52 havingan activated charcoal matrix which ensures that no odours escape withthe gas exhausted from the device 32 via an exhaust port 54. The filter52 material also serves as noise reducing material to enhance the effectof the silencer system 50. The device 32 also contains a battery pack 56to power the apparatus which battery pack also powers the control system60 which controls a user interface system 62 controlled via a keypad(not shown) and the aspiration pump 44 via signals from sensors 46, 48.A power management system 66 is also provided which controls power fromthe battery pack 56, the recharging thereof and the power requirementsof the aspirant pump 44 and other electrically operated components. Anelectrical connector 68 is provided to receive a power input jack 70from a SELV power supply 72 connected to a mains supply 74 when the userof the apparatus or the apparatus itself is adjacent a convenient mainspower socket.

FIG. 2 shows a similar schematic representation to FIG. 1 but shows thefluid paths in more detail. The wound exudate is aspirated from thewound site/dressing 14 via the conduit 12, the two connector portions18, 20 and the conduit 24 into the waste canister 22. The waste canister22 comprises a relatively large volume 80 in the region of 500 ml intowhich exudate from the wound is drawn by the aspiration system at anentry port 82. The fluid 84 drawn into the canister volume 80 is amixture of both air drawn into the dressing 14 via the semi-permeableadhesive sealing drape (not shown) and liquid 86 in the form of woundexudates. The volume 80 within the canister is also at a loweredpressure and the gaseous element 88 of the aspirated fluids is exhaustedfrom the canister volume 80 via the filters 26 and the waste canisterexhaust exit port 28 as bacteria-free gas. From the exit port 28 of thewaste canister to the final exhaust port 54 the fluid is gaseous only.

FIG. 3 shows a schematic diagram showing only the device portion of theapparatus and the power paths in the device of the apparatus embodyingthe present invention. Power is provided mainly by the battery pack 56when the user is outside their home or workplace, for example, however,power may also be provided by an external mains 74 supplied chargingunit 72 which when connected to the device 32 by the socket 68 iscapable of both operating the device and recharging the battery pack 56simultaneously. The power management system 66 is included so as to beable to control power of the TNP system. The TNP system is arechargeable, battery powered system but is capable of being rundirectly from mains electricity as will be described hereinafter morefully with respect to the further figures. If disconnected from themains the battery has enough stored charge for approximately 8 hours ofuse in normal conditions. It will be appreciated that batteries havingother associated life times between recharge can be utilised. Forexample batteries providing less than 8 hours or greater than 8 hourscan be used. When connected to the mains the device will run off themains power and will simultaneously recharge the battery if depletedfrom portable use. The exact rate of battery recharge will depend on theload on the TNP system. For example, if the wound is very large or thereis a significant leak, battery recharge will take longer than if thewound is small and well sealed.

FIG. 4 shows the device 32 part of the apparatus embodying the presentinvention and the data paths employed in the control system for controlof the aspirant pump and other features of the apparatus. A key purposeof the TNP system is to apply negative pressure wound therapy. This isaccomplished via the pressure control system which includes the pump anda pump control system. The pump applies negative pressure; the pressurecontrol system gives feedback on the pressure at the pump head to thecontrol system; the pump control varies the pump speed based on thedifference between the target pressure and the actual pressure at thepump head. In order to improve accuracy of pump speed and hence providesmoother and more accurate application of the negative pressure at awound site, the pump is controlled by an auxiliary control system. Thepump is from time to time allowed to “free-wheel” during its duty cycleby turning off the voltage applied to it. The spinning motor causes a“back electro-motive force” or BEMF to be generated. This BEMF can bemonitored and can be used to provide an accurate measure of pump speed.The speed can thus be adjusted more accurately than can prior art pumpsystems.

According to embodiments of the present invention, actual pressure at awound site is not measured but the difference between a measuredpressure (at the pump) and the wound pressure is minimised by the use oflarge filters and large bore tubes wherever practical. If the pressurecontrol measures that the pressure at the pump head is greater than atarget pressure (closer to atmospheric pressure) for a period of time,the device sends an alarm and displays a message alerting the user to apotential problem such as a leak.

In addition to pressure control a separate flow control system can beprovided. A flow meter may be positioned after the pump and is used todetect when a canister is full or the tube has become blocked. If theflow falls below a certain threshold, the device sounds an alarm anddisplays a message alerting a user to the potential blockage or fullcanister.

Referring now to FIGS. 5 to 9 which show various views and crosssections of a preferred embodiment of apparatus 200 embodying thepresent invention. The preferred embodiment is of generally oval shapein plan and comprises a device unit 202 and a waste canister 204connected together by catch arrangements 206. The device unit 202 has aliquid crystal display (LCD) 208, which gives text based feedback on thewound therapy being applied, and a membrane keypad 210, the LCD beingvisible through the membrane of the keypad to enable a user to adjust orset the therapy to be applied to the wound (not shown). The device has alower, generally transverse face 212 in the centre of which is a spigot214 which forms the suction/entry port 216 to which the aspiration means(to be described below) are connected within the device unit. The loweredge of the device unit is provided with a rebated peripheral malemating face 218 which engages with a co-operating peripheral femaleformation 220 on an upper edge of the waste canister 204 (see FIGS. 8and 9). On each side of the device 202, clips 222 hinged to the canister204 have an engaging finger (not shown) which co-operates withformations in recesses 226 in the body of the device unit. From FIG. 7it may be seen that the casing 230 of the device unit is of largely“clamshell” construction comprising front and back mouldings 232, 234,respectively and left-hand and right-hand side inserts 236, 238. Insidethe casing 230 is a central chassis 240 which is fastened to an internalmoulded structural member 242 and which chassis acts as a mounting forthe electrical circuitry and components and also retains the batterypack 246 and aspiration pump unit 248. Various tubing items 250, 252,254 connect the pump unit 248 and suction/entry port 216 to a finalgaseous exhaust via a filter 290. FIG. 8 shows a partially sectionedside elevation of the apparatus 200, the partial section being aroundthe junction between the device unit 202 and the waste canister 204, across section of which is shown at FIG. 9. Theses views show the rebatededge 218 of the male formation on the device unit co-operating with thefemale portion 220 defined by an upstanding flange 260 around the topface 262 of the waste canister 204. When the waste canister is joined tothe device unit, the spigot 214 which has an “O” ring seal 264therearound sealingly engages with a cylindrical tube portion 266 formedaround an exhaust/exit port 268 in the waste canister. The spigot 214 ofthe device is not rigidly fixed to the device casing but is allowed to“float” or move in its location features in the casing to permit thespigot 214 and seal 264 to move to form the best seal with the bore ofthe cylindrical tube portion 266 on connection of the waste canister tothe device unit. The waste canister 204 in FIG. 9 is shown in an uprightorientation much as it would be when worn by a user. Thus, any exudate270 would be in the bottom of the internal volume of waste receptacleportion 272. An aspiration conduit 274 is permanently affixed to anentry port spigot 278 defining an entry port 280 to receive fluidaspirated from a wound (not shown) via the conduit 274. Filter members282 comprising a 0.2 μm filter and 284 comprising a 1 μm filter arelocated by a filter retainer moulding 286 adjacent a top closure memberor bulkhead 288 the filter members preventing any liquid or bacteriafrom being drawn out of the exhaust exit port 268 into the pump andaspiration path through to an exhaust and filter unit 290 which isconnected to a casing outlet moulding at 291 via an exhaust tube (notshown) in casing side piece 236. The side pieces 236, 238 are providedwith recesses 292 having support pins 294 therein to locate a carryingstrap (not shown) for use by the patient. The side pieces 230 andcanister 204 are also provided with features which prevent the canisterand device from exhibiting a mutual “wobble” when connected together.Ribs (not shown) extending between the canister top closure member 288and the inner face 300 of the upstanding flange 260 locate in grooves302 in the device sidewalls when canister and device are connected. Thecasing 230 also houses all of the electrical equipment and control andpower management features, the functioning of which was describedbriefly with respect to FIGS. 3 and 4 hereinabove. The side piece 238 isprovided with a socket member 298 to receive a charging jack from anexternal mains powered battery charger (both not shown).

FIG. 10 shows a schematic cross section of a wound 400 and apparatus 402connected thereto to apply TNP therapy to the wound and to measure itsvolume. The apparatus comprises wound filling material 404; an aspirantconduit 406 having one end 408 sealed within the wound cavity 410 by anoverlying sealing drape 412 at point 414 and a distal end 416 of theconduit 406 attached to a waste canister 418 for the collection ofexudate from the wound. The waste canister is operably connected to apressure sensor 420 by a conduit portion 422 and to the inlet side of avacuum pump 426 by a conduit portion 428. The vacuum pump output side isconnected by a conduit portion 430 to a flow sensor 432 and exhaust istaken away via a conduit 434. A control and monitoring system isprovided at 440 and receives signals 442 from the pressure sensor 420;receives signals 444 from the vacuum pump; and, receives signals 446from the flow sensor 432. The control system 440 sends control signals448 to the vacuum pump 426 in order for it to maintain a steady statepressure as set by instructions 450 entered by a clinician/user on adata entry keypad 452. The control system 440 outputs data 454 to adisplay 456 such as an LCD display, for example, as required by theclinician/user in response to instructions 450 but includes datarelating to the volume of the wound 400. The sealing drape 412 is sealedto the patient's sound flesh at 460 surrounding the wound 400 generallyby a layer of pressure sensitive adhesive (not shown) on the fleshcontacting side of the drape 412.

In operation a clinician/user enters a required pressure, consistentwith calibration procedures previously employed, to be achieved in thewound cavity 410 on the keypad 452. The vacuum pump 426 is initiated andpumps the wound cavity 410 down to the required pressure and continuesto operate as appropriate to maintain the required pressure at a steadystate. The apparatus 402 has been calibrated in that the volumes of theconduit 406; empty waste canister 418; conduits 422, 428, 430; andvacuum pump 426 are known and a correction factor has been entered inthe memory of the control system 440.

FIG. 11 shows a graph of fluid flow, in this case air against time toachieve steady state conditions. FIG. 11 indicates time taken to reach asteady state pressure as set in the control system at start up and whichtime to reach a steady state pressure is indicated by the line 500.Initially the rate of air flow rises rapidly to a maximum at 502 afterwhich the flow rate falls off as the set pressure is approached and thesteady state is achieved at line 502. Part of the flow rate under thecurve 504 may probably be due to an inward leak of air, most probablyinto the wound cavity 410 (the joints between the component parts of theapparatus from the conduit 406 in the direction of fluid flow should allbe sound and leak-free) between the sealing drape 412 and the patient'ssound flesh 460. The point 508 indicates the steady state flow rate tomaintain the desired set pressure and the area of the triangle 510, 508,512 indicates the portion or volume of air aspirated from the woundcavity attributable to the leak and may be deducted from the total areaunder the curve 504 up to the line 500. Thus, the control and monitoringsystem 440 computes the volume of the wound from the area under thecurve 504 bounded by the line 508-510 and will be displayed on thedisplay 456 and/or in suitable recording apparatus (not shown).

A similar graph to that of FIG. 11 and shown in FIG. 12 may be drawn butshowing flow against pressure in the wound cavity 410. Steady state isreached when the set pressure has been achieved. In the graph of FIG. 12the set pressure 600 and steady state coincide at the line 602 and thetotal area under the curve 604 is comprised of: a factor relating toleak rate as before and is the triangular area 600, 606, 608; a factorrelating to system volume indicated by the vertically hatched area 610and the wound volume indicated by the area 612. When the system is at asteady state indicated by the line 614 the flow through the flow sensorrelates only to the leakage into the system.

The volume of the wound may be computed from the expression:

$V_{wound} = {{Area}\mspace{14mu}{under}\mspace{14mu}{curve} \times \frac{\left( {760 - {{Set}\mspace{14mu}{pressure}\mspace{14mu}{in}\mspace{14mu}{mmHg}}} \right)}{760}}$

If the dressing drape 412 is completely leak-free then the steady stateflow rate when the set required pressure is reached will be zero and thepoint 508 will lie on the time axis.

The flow meter 432 is shown as situated after the vacuum pump 426 butmay in other embodiments of apparatus according to the present inventionlie between the waste canister 418 and the vacuum pump 426. Suitablecorrection factors relating to the exact arrangement of apparatus usedwill need to be determined.

The method of the present invention may be carried out by the apparatusdescribed and discussed with reference to FIGS. 1 to 9 above which alsoconstitutes apparatus according to the present invention.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

What is claimed is:
 1. A negative pressure wound therapy, apparatuscomprising: a source of negative pressure configured to provide negativepressure, via a fluid flow path, to a wound dressing placed over awound; and a controller programmed to operate the source of negativepressure, the controller further programmed to: measure a volume of airextracted from at least the wound; measure a leak rate in the fluid flowpath; determine a wound volume based at least in part on the measuredvolume of air; and correct the determined wound volume based at least inpart on the measured leak rate in the fluid flow path.
 2. The apparatusaccording to claim 1 wherein the controller is further configured tooperate the source of negative pressure to achieve a steady state levelof negative pressure under the dressing.
 3. The apparatus of claim 1,wherein the controller is configured to measure the leak rate based atleast in part on a flow rate measured after reaching a steady statelevel of negative pressure under the dressing.
 4. The apparatus of claim3, further comprising a pressure sensor configured to measure pressurein the fluid flow path.
 5. The apparatus of claim 4, wherein thepressure sensor is positioned in the fluid flow path between the sourceof negative pressure and a waste canister.
 6. The apparatus of claim 1,further comprising a flow sensor configured to measure the volume of airextracted from at least the wound.
 7. The apparatus of claim 6, whereinthe flow sensor comprises a flow meter.
 8. The apparatus of claim 1,wherein the controller is further configured to measure a volume of atleast one component located in the fluid flow path to produce acorrection factor for determining the wound volume.
 9. The apparatus ofclaim 8, wherein the at least one component located in the fluid flowpath comprises at least one of a waste canister, one or more conduits,and the source of negative pressure.
 10. The apparatus of claim 9,wherein the controller is further configured to measure the volume ofthe waste canister when the waste canister is empty.
 11. The apparatusof claim 8, wherein the controller is further configured to subtract thecorrection factor from the measured volume of air extracted from atleast the wound.
 12. The apparatus of claim 2, wherein the controller isfurther configured to determine the wound volume and correct thedetermined wound volume when the wound dressing is bein changed.
 13. Theapparatus of claim 1, wherein the controller is further configured tocompute, based at least in part on the leak rate, a volume of airextracted from at least the wound due to a leak and correct thedetermined wound volume by subtracting the volume of air extracted fromat least the wound due to the leak from the measured volume of airextracted from at least the wound.
 14. The apparatus of claim 1, furthercomprising a display, wherein the controller is further configured toprovide the cause the corrected wound volume to be displayed on thedisplay.